Considerable effort has been devoted to reducing the sidewall roughness to improve the performance of optical components. Techniques include anisotropic wet etching, thermal oxidation and combinations of the two. However, these methods are either limited to certain crystalline facets of semiconductor materials, or often involve harsh processing conditions, high temperatures or complicated procedures, which could degrade the substrate and other existing devices on the chip.

To solve these issues, researchers at Princeton University, US, have come up with a way of selectively smoothing the sidewall roughness of waveguides without stress or damage to either the existing devices or the substrate. Even better, the method is a self-perfection process and is simple to implement.

Self-perfection by liquefaction

Self-perfection by liquefaction (SPEL) is performed using a XeCl excimer laser with a wavelength of 308 nm and a pulse duration of 20 ns. The laser spot is about 3 mm × 3 mm and the laser pulse energy can be changed by adjusting a variable attenuator to selectively melt the surface layer of the waveguide. In a molten state, materials such as silicon have an extremely low viscosity (lower than the water), and can flow and smooth out the edge roughness by themselves under surface tension.

Using the technique, the Princeton researchers have reduced the average sidewall roughness of a silicon waveguide from 13 to 3 nm (1σ). According to their calculations, this is equivalent to reducing light loss from 53 to 3 dB/cm – a five orders of magnitude increase in light transmission for a 1 cm long waveguide.

Tests show that the new approach can fix defects within just 200 ns. This short exposure time together with SPEL's material selectivity, allows developers to repair defective components on a chip without damaging surrounding components and materials, and makes SPEL a promising candidate for enabling higher-density nanophotonics.

The researchers presented their work in Nanotechnology.